In the osteosynthetic treatment of fractures of the neck of the femur, that is, fractures of the femur in which the neck connecting the head with the rest of the femur is broken, the bone fragments are connected to one another by means of a temporarily inserted fixation device. Such devices, as described, for example, in U.S. Pat. No. 4,095,591, consist essentially of:
an anchor bolt or lag screw to be screwed into the head of the bone, and having a headless shaft with an internally threaded socket; PA1 a plate, to be attached to the main section or shank of the femur, with a barrel which serves to hold the shaft of the anchor bolt; and PA1 a compression screw to be screwed into the threaded socket of the anchor bolt shaft, and having a head which, in position, rests on the shoulder of the barrel. In the profession, these devices are known as dynamic hip screws.
The operating technique for implanting such fixation devices is described in Swiss patents CH-A5 634.741 and 634.742. In general the anchor bolt is inserted into the head of the femur, traversing the fracture. The plate is then attached to the shank of the femur with its barrel over the shaft of the bolt. The compression screw is thereupon inserted into the socket in the anchor bolt, the arrangement being such that the compression screw is retained in its position by engagement with the barrel of the compression plate so that when screwed into the anchor bolt, it retracts the bolt, reducing the fracture.
Anchor bolts used in such devices have to date had external threads whose profiles are typical of bone screws and thoroughly conventional.
Such conventional profiles create maximum resistance to tractive forces acting on the bolt, because of their approximately saw-tooth profile, which faces the end of the bolt shaft. However, in the use under discussion here, because of the ability of the anchor bolt to slide in the plate socket, only limited tractive forces act on the anchor bolt, so that the conventional profile is not appropriate. In contrast, the resistance of the conventional profiles against pressure forces that occur precisely in this use of such anchor bolts is rather limited.
A further disadvantage of the conventional screw profiles is that because of their sharp threading they offer practically no resistance to lateral pressures. Precisely such lateral forces occur, however, in the physiological load on the operated hip joint head in question. In the case of porotic femur heads, these lateral forces can lead to a penetration of the thread into the joint. The reason is that part of the spongiosa is missing, so that the bolt can be anchored only in the thin cortical skin of the femur head. In other words, only the thread tips carry the laterally occurring load on the bolt. The problem of bolt penetration into the hip joint is very relevant, and is described, for example, by A. H. R. W. Simpson et al in British Journal of Accident Surgery, Vol. 20, No. 4, July 1989. This article reports that in a total of 223 cases studied, 14 bolts penetrated the hip joint, and, with 12 bolts there was superior penetration of the hip head.